(1) Field of the Invention
The present invention relates to a steelmaking process in a converter using a great or substantial amount of a so-called solid iron-bearing cold material such as cold iron, pellet, scrap and the like under oxygen blowing. More particularly, the invention relates to a steelmaking-refining technique which permits a cheaper production of a high quality steel free from a contamination of a molten steel due to resulfurization, re-phosphorization and so on and also conspicuously prolongs a converter durable life.
(2) Description of the Prior Art
According to the usual converter refining, it is a common practice that a relatively small amount of scrap is added to a molten iron supplied from a blast furnace, and a slag-forming agent and a high flow rate of oxygen are supplied thereto to carry out oxygen-blowing.
In this usual converter refining, the amount of the scrap for the total amount of all the materials charged into the converter is limited to 20% due to the restriction of a heating source.
Thus, a large amount of the scrap cannot be used in this process.
For overcoming this restriction, as the steelmaking process in which a large amount of the iron-bearing cold material which is in the total amount or in the major part the scrap or cold iron is used and such an iron-bearing cold material is refined by means of a converter, there is a technique as proposed in W. German Pat. No. 28 38 983 that the so-called scrap is charged into the converter, and is preheated by using nozzles installed at the bottom of the converter as burner and then completely melted, if necessary by adding a small amount of molten iron; a carbonaceous material such as coke powder or the like is blown into the converter through the nozzles of the converter bottom, and at the same time oxygen is blown from a lance or the bottom nozzles, thereby refining the molten iron.
However, according to this technique, since the heat efficiency is low, it takes an extremely long time to preheat and completely melt the cold material. Further, even if the refining is started, it also takes a long time to complete refining. Therefore, this process has defects in that it is in no harmony with the operation schedule of the succeeding continuous casting apparatus making the synchronization difficult and in that the damage to the converter wall is vigorous to lower the durable life of the converter.
In addition, Japanese Patent Laid-Open Application No. 56(1981)-58,916, discloses a process of making steel from a solid iron-bearing material by using a converter provided with oxygen top-blowing devices and carbonaceous material introducing nozzles, in which a molten steel of an excessive amount of 10-30% than the amount per one charge is produced from scrap by refining and the excessive amount of molten steel remains in the converter and the next scrap is charged therein and the succeeding refining is carried out.
However, according to this process, a part of the molten steel is consecutively left for the succeeding steps in the same converter, and therefore, it has the following defects:
(i) A sulfur component in the carbonaceous material such as coal (S: 0.44%, ash: 13.7%, VM: 13.1%) and coke (S: 0.57%, ash: 17.1%) used in a large amount is included in the molten steel. PA0 (ii) Since there exists a wide range of temperature change from the scrap-melting to decarbonization-refining, and the temperature becomes high at the time of discharging, the durable life of a refractory material is short. PA0 (iii) Since the continuous refining is performed from the scrap-melting to the blowing-termination in a single converter, the same is used for a long period of time. Therefore, a large amount of hydrogen from heat decomposition of propane or the like used for protecting (cooling) the bottom-blown tuyeres is absorbed into the molten steel.